Current video card design should be compatible with most or all motherboards today (besides the type of interface is concerned). I have a 7800GT and that sucker is suprisingly long. Much longer than my 6600GT. The end of the board hovers over one of my chipset fan, but it's not really obstructing anything because the video card is so thin. Thin but long and powerful. The only way I can see that it doesn't fit is if you were to install aftermarket heatsink/fan combo for the chipset. Then it may not fit depending how high the fan sits. I haven't seen the picts for that DFI motherboard you have in mind but stockwise, they're pretty low. I think they're just a fan with heatsink features slapped on a chip instead of an actual heatsink plus a fan on top.
As for the soundcard, here's my take on it. X-Fi soundcards are known for performance. They have to keep in mind that people want 5.1 surround capabilities so there's no way Creative Labs would make sound card that good and not be 5.1 compatible. So despite having just 4-ports, it's enough for every type of connections available. I guess I should break out my Audigy2 retail box with the manual inside to see what all 6-ports do, but it's tucked away deep in my closet. I'm using one right now, but only 3 of them are being used. An X-Fi has 4 and most of the time, you'll probably just use 3 of them instead so there's really no need to have more ports on it. Maybe Creative Labs found a way to streamline the card so it works best with just 4 inputs instead of 6.